A passerine is a bird of the order Passeriformes, which includes more than half of all bird species. Sometimes known as perching birds or, less accurately, as songbirds, the passerines form one of the most diverse terrestrial vertebrate orders: with over 5,000 identified species,[1] it has roughly twice as many species as the largest of the mammal orders, the Rodentia. It contains over 110 families, the second most of any order of vertebrates (after the Perciformes).

The names "passerines" and "Passeriformes" are derived from Passer domesticus, the scientific name of the type species—the House Sparrow—and ultimately from the Latin term passer for Passer sparrows and similar small birds.

Characteristics

The order is divided into three suborders, Tyranni (suboscines), Passeri (oscines), and the basal Acanthisitti. Oscines have the best control of their syrinx muscles among birds, producing a wide range of songs and other vocalizations (though some of them, such as the crows, do not sound musical to human beings); some such as the lyrebird are accomplished imitators. The Acanthisittids or New Zealand wrens are tiny birds restricted to New Zealand, at least in modern times; they were long placed in Passeri; their taxonomic position is uncertain, though they seem to be a distinct and very ancient group.

Most passerines are smaller than typical members of other avian orders. The heaviest and altogether largest passerines are the Thick-billed Raven and the larger races of Common Raven, each exceeding 1.5 kg (3.3 lbs) and 70 cm (28 in). The Superb Lyrebird and some birds-of-paradise, to 110 cm (43 in) due to very long tails or tail coverts, are longer overall.[verification needed] The smallest passerine is the Short-tailed Pygmy-Tyrant, at 6.5 cm (2.5 in) and 4.2 g (0.14 oz).
Pterylosis or the feather tracts in a typical passerine

The foot of a passerine has three toes directed forward and one toe directed backwards, called anisodactyl arrangement. This arrangement enables the passerine birds to perch upon vertical surfaces, such as trees and cliffs. The toes have no webbing or joining, but in some cotingas the second and third toes are united at their basal third. The hind toe joins the leg at the same level as the front toes. In other orders of birds the toe arrangement is different. The leg muscle of passerine birds contains a special adaption for perching. It will automatically tighten and become stiff, if the bird starts to lose hold of the branch on which it is perching. This enables passerine to sleep while perching without falling off. This is especially useful for passerine birds that develop nocturnal lifestyles.[2]

Most passerine birds develop twelve tail feathers, though the Superb Lyrebird has sixteen.[citation needed] Certain species of passerines have stiff tail feathers, which help the birds balance themselves when perching upon vertical surfaces.

The chicks of passerines are altricial; blind, featherless, and helpless when hatched from their eggs. This requires that the chicks receive a lot of parental care. Most passerines lay coloured eggs, in contrast with non-passerines, most of whose eggs are white except in some ground-nesting groups such as Charadriiformes and nightjars, where camouflage is necessary, and some parasitic cuckoos, which match the passerine host's egg.

Origin and evolution

The evolutionary history of the passerine families and the relationships among them remained rather mysterious until the late 20th century. In many cases, passerine families were grouped together on the basis of morphological similarities which, it is now believed, are the result of convergent evolution, not a close genetic relationship. For example, the "wrens" of the northern hemisphere, those of Australia, and those of New Zealand look very similar and behave in similar ways, and yet belong to three far-flung branches of the passerine family tree; they are as unrelated as it is possible to be while remaining Passeriformes.[3]

Much research remains to be done, but advances in molecular biology and improved paleobiogeographical data are gradually revealing a clearer picture of passerine origins and evolution that reconciles molecular affinities, the constraints of morphology and the specifics of the fossil record.[4] It is now thought that the first passerines evolved in Gondwana at some time in the Paleogene, maybe around the Late Paleocene some 60–55 mya.[5] The initial split was between the Tyranni, the songbirds, the Eurylaimides and the New Zealand "wrens", which must have diverged during a short period of time (some million years at most). The Passeriformes apparently evolved out of a fairly close-knit clade of "near passerines" which contains such birds as the Piciformes and Coraciiformes.[6]

A little later, a great radiation of forms took place out of Australia-New Guinea: the Passeri or songbirds. A major branch of the Passeri, "Parvorder Passerida", emerged either as the sister group to the basal lineages and corvoids ("Parvorder Corvida"), or more likely as a subgroup of it, and expanded deep into Eurasia and Africa, where there was a further explosive radiation of new lineages. This eventually led to three major passeridan lineages comprising about 4,000 species, which in addition to the corvoidan clade and numerous minor lineages make up songbird diversity today. There has been extensive biogeographical mixing, with northern forms returning to the south, southern forms moving north, and so on.

Fossil record

Earliest passerines

Perching bird osteology, especially of the limb bones, is rather diagnostic.[7] However, the early fossil record is poor because the first Passeriformes were apparently on the small side of the present size range, and their delicate bones did not preserve well. QM specimens F20688 (carpometacarpus) and F24685 (tibiotarsus) from Murgon, Queensland are fossil bone fragments clearly recognizable as passeriform; they represent two species of approximately 10 and 20 cm in overall length and prove that some 55 mya, barely into the Early Eocene, early perching birds were recognizably distinct.[8]
Male Superb Lyrebird (Menura novaehollandiae). This very primitive songbird shows strong sexual dimorphism, with a peculiarly apomorphic display plumage in males.

A quite similar group, the Zygodactylidae (named for their zygodactylous approach to perching) independently arose at much the same time – and possibly from closely related ancestors – in the landmasses bordering the North Atlantic, which at that time was only some two-thirds of its present width.

Until the discovery of the Australian fossils Palaeospiza bella from the Priabonian Florissant Fossil Beds (Late Eocene, around 35 mya) was the oldest known passeriform. However, it is now considered a non-passeriform near passerine.

From the Bathans Formation at the Manuherikia River in Otago, New Zealand, MNZ S42815 (a distal right tarsometatarsus of a Tui-sized bird) and several bones of at least one species of Saddleback-sized bird have recently been described. These date from the Early to Middle Miocene (Awamoan to Lillburnian, 19-16 mya).[9]

Modern knowledge about the living passerines' interrelationships (see the list of families below) suggests that the last common ancestor of all living Passeriformes was a small forest bird, probably with a stubby tail[10] and an overall drab coloration, but possibly with marked sexual dimorphism. The latter trait seems to have been lost and re-evolved multiple times in songbird evolution alone, judging from its distribution among the extant lineages. Sexual dichromatism is very rare among the basal lineages of Passerida, and probably their plesiomorphic condition. But among the youngest passerid clade, the Passeroidea, extremely colorful males and drab females are common, if not the rule. On the other hand, among the basalmost Passeri there are a considerable number of strongly dimorphic lineages too, such as the very ancient Menuridae as well as many Meliphagoidea and Corvoidea. Sexual dimorphism is also not uncommon in the Acanthisittidae and prominent in some suboscines such as the Pipridae and Cotingidae.

Early European passerines

In Europe, perching birds are not too uncommon in the fossil record from the Oligocene onwards, but most are too fragmentary for a more definite placement:

Wieslochia was possibly not a member of any extant suborder. That not only the Passeri expanded much beyond their region of origin is proven by an undetermined broadbill (Eurylaimidae) from the Early Miocene (roughly 20 mya) of Wintershof, Germany, and the indeterminate Late Oligocene suboscine from France listed above. Even very basal Passeriformes might have been common in Europe until the Middle Miocene, some 12 mya.[17] Extant Passeri superfamilies were quite distinct by that time and are known since about 12–13 mya when modern genera were present in the corvoidean and basal songbirds. The modern diversity of Passerida genera is known mostly from the Late Miocene onwards and into the Pliocene (about 10–2 mya). Pleistocene and early Holocene lagerstätten (<1.8 mya) yield numerous extant species, and many yield almost nothing but extant species or their chronospecies and paleosubspecies.

American fossils

In the Americas, the fossil record is more scant before the Pleistocene, from which several still-existing suboscine families are documented. Apart from the indeterminable MACN-SC-1411 (Pinturas Early/Middle Miocene of Santa Cruz Province, Argentina),[18] an extinct lineage of perching birds has been described from the Late Miocene of California, USA: the Palaeoscinidae with the single genus Paleoscinis. "Palaeostruthus" eurius (Pliocene of Florida) probably belongs to an extant family, most likely passeroidean.

See also Late Quaternary prehistoric birds.

Systematics and taxonomy

Initially,
the Corvida and Passerida were classified as "parvorders" in the suborder Passeri; in accord with the usual taxonomic practice, they would probably be ranked as infraorders. As originally envisioned in the Sibley-Ahlquist taxonomy, they contained, respectively, the large superfamilies Corvoidea and Meliphagoidea as well as minor lineages, and the superfamilies Sylvioidea, Muscicapoidea and Passeroidea.
The peculiar Bearded Reedling, Panurus biarmicus, may be the most enigmatic passerine. No truly close relatives have been identified.

This arrangement has been found to be overly simplified by more recent research. Since the mid 2000s, literally dozens of studies are being published which try rather successfully to resolve the phylogeny of the passeriform radiation. For example, the Corvida in the traditional sense were a rather arbitrary assemblage of early and/or minor lineages of passeriform birds of Old World origin, generally from the region of Australia, New Zealand, and Wallacea. The Passeri on the other hand can be made monophyletic by moving some families about, but the "clean" three-superfamily-arrangement has turned out to be far more complex and it is uncertain whether future authors will stick to it.

Major "wastebin" families such as the Old World warblers and Old World babblers have turned out to be paraphyletic and are being rearranged. Several taxa turned out to represent highly distinct species-poor lineages and consequently new families had to be established, some of them – like the Stitchbird of New Zealand and the Eurasian Bearded Reedling – monotypic with only one living species.[19] It seems likely that in the Passeri alone, a number of minor lineages will eventually be recognized as distinct superfamilies. For example, the kinglets constitute a single genus with less than 10 species today, but seem to have been among the first perching bird lineages to diverge as the group spread across Eurasia. No particularly close relatives of them have been found among comprehensive studies of the living Passeri, though it is suspected that they might be fairly close to some little-studied tropical Asian groups. Treatment of the nuthatches, wrens, and their closest relatives as a distinct superfamily Certhioidea is increasingly considered justified; the same might eventually apply to the tits and their closest relatives.

This process is still continuing. Therefore, the arrangement as presented here is subject to change. However, it should take precedence over unreferenced conflicting treatments in family, genus and species articles here; see the next section for default sources.

Taxonomic list of Passeriformes families

This list is in taxonomic order, placing related species/groups next to each other. The Passerida subdivisions are updated as needed from the default sequence of the Handbook of the Birds of the World,[20] based on the most modern and comprehensive studies.[21]

Regarding arrangement of families

The families are sorted into a somewhat unusual sequence. This is because so many reallocations have taken place since about 2005 that a definite arrangement has not been established yet. The present sequence is an attempt to preserve as much of the traditional sequence while giving priority to adequately addressing the relationships between the families.
[edit] Suborder Acanthisitti

* Superfamily Corvoidea – a highly diverse group of global distribution, but most plentiful in the Australasian region and surroundings. The oldest truly globally successful group of passerines, they include among them what may well be the most intelligent and the most spectacular of the order.

* Superfamily Sylvioidea – mostly insectivores, distribution centered on the Indo-Pacific region. Few occur in the Australian region and fewer still in the Americas. Usually sleek and drab birds, few have pronounced sexual dimorphism.

* Superfamily Muscicapoidea – mostly insectivores, near-global distribution centered on Old World tropics. One family endemic to Americas. Nearly absent (except introductions) from the Australian region. Usually rather stocky for their size, most are quite dark and dull though Sturnidae are commonly iridescent and/or colorful. Sexual dimorphism often absent, sometimes pronounced.
o Cinclidae: dippers
o Muscicapidae: Old World flycatchers and chats. Monophyly needs confirmation.
o Turdidae: thrushes and allies. Monophyly needs confirmation.
o Buphagidae: oxpeckers. Formerly usually included in Sturnidae.
o Sturnidae: starlings and possibly Philippine creepers. Placement of latter in Muscicapoidea seems good, but inclusion in Sturnidae requires confirmation; possibly distinct family Rhabdornithidae.
o Mimidae: mockingbirds and thrashers

* Superfamily Certhioidea – wrens and allies. Sometimes included in Muscicapoidea.
o Sittidae: nuthatches
o Tichodromadidae: Wallcreeper: Traditionally placed as a subfamily of the nuthatches and more rarely of the treecreepers, no study has been able to verify either placement this far. Thus it is better considered a monotypic family, at least for the time being.
o Certhiidae: treecreepers
o Salpornithidae: Spotted Creeper. Tentatively placed here; often considered a subfamily of the Certhidae.
o Troglodytidae: wrens
o Polioptilidae: gnatcatchers

* Passerida incertae sedis – Rather basal Passerida, most of which seem to constitute several small but distinct lineages that could be considered superfamilies. Most occur in Asia, Africa and North America.
o Panurus: Bearded Reedling (Bearded "Tit"). Relationships enigmatic. Formerly in "Paradoxornithidae", might be included in Sylvioidea as monotypic family Panuridae or even constitute the smallest passerine superfamily.
o Possible superfamily Paroidea – titmice and allies. Might be included in Sylvioidea.